Process for preparing nylon salt and its polymerization

ABSTRACT

The present invention relates to a process for preparing a granular nylon salt material, wherein an aqueous mixture, comprising at least 65 wt. % of salt components, and at most 35 wt. % of an aqueous medium, the weight percentages (wt. %) being relative to the total weight of the aqueous mixture, is flashed from a pressure vessel via a flash valve into a flash chamber, thereby evaporating the aqueous medium in an amount sufficient to result in a residual moisture content of at most 7.5 wt. %, and forming a granular nylon salt material. The invention also relates to a process for preparing a nylon polymer, comprising direct solid state polymerization of the granular salt material prepared by flash granulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International ApplicationNo. PCT/EP2015/066980 filed 24 Jul. 2015, which designated the U.S. andclaims priority to EP Patent Application No. 14178953.7 filed 29 Jul.2014, the entire contents of each of which are hereby incorporated byreference.

FIELD

The present invention relates to a process for preparing a nylon salt.The present invention also relates to a process for preparing apolyamide by polymerization of a nylon salt.

BACKGROUND AND SUMMARY

A nylon salt is herein understood to be a diammonium dicarboxylate salt,also indicated as a diamine/dicarboxylic acid salt, in other words asalt prepared from a diamine, or a mixture of diamines, and adicarboxylic acid or a mixture of dicarboxylic acids. Polyamidesprepared from such salts are generally indicated as AA-BB polyamides.Herein an AA-BB polyamide is understood to be a polyamide comprisingalternating AA and BB repeating units, wherein AA represents therepeating units derived from diamine and BB represents the repeatingunits derived from dicarboxylic acid. Throughout this text the wordings“diammonium dicarboxylate salt” and “nylon salt” are meant to have thesame meaning and are therefore interchangeable.

For the preparation of polyamides from nylon salts, different processesare known. The known processes include melt processes, solutionprocesses and solid state processes. Known solid state processes includeprocesses comprising solid state post condensation of polyamideprepolymer obtained by melt polymerization, solution polymerizationfollowed by flashing and solid state post condensation and direct solidstate polymerization processes. In such a process, the salt used isgenerally a granular material, such a powder, and the aim is also toobtain the resulting polymer as a granular material. The salt materialused herein can be a salt powder or granular material obtained, forexample, by spray drying, precipitation from solution, or a dry routeprocess involving reaction of liquid diamine with solid dicarboxylicacid. The salt may also have a particular shape of compacted powderparticles. Such a process is described, for example, in the U.S. Pat.No. 5,128,442 and GB-801,733.

In a melt process, a diamine, or a mixture of diamines, and adicarboxylic acid or a mixture of dicarboxylic acids are mixed whereinat least one of the diamine or the dicarboxylic acid is in a melt formand the mixture of the two is obtained in a form of a melt. Theresulting melt can be further polymerized to obtain a polyamide polymer.

Not all nylon salts are suited to be prepared in the melt, or even to bepolymerized in the melt. In particular polyamides with high meltingtemperatures, such semi-crystalline semi-aromatic polyamides, tend togive side reactions when prepared in the melt.

In a solution process the diamine(s) and dicarboxylic acid(s) are firstdissolved in a solvent. The solvent suitably consists of water orcomprises a mixture of water and an organic co-solvent. The salt may beeither isolated from the solution, and then polymerized by means of asuitable polymerization process, or first converted in the solution intoa prepolymer and then isolated from the solution, and furtherpolymerized by solid-state post condensation, or be converted into aprepolymer in the solution and then concentrated by evaporation of waterand further polymerized in the melt.

Salts isolated from solution may be used for polymerization intopolyamides. Known methods for the polymerization of isolated saltsinclude melt polymerization, optionally followed by solid-state postcondensation, and direct solid state polymerization.

For the isolation from solution different processes are known, includingspray drying, crystallization induced by cooling followed byprecipitation, precipitation induced by adding an organic non-solvent,or induced by evaporation of water and any co-solvent present.

Typically, the organic co-solvent as well as the organic non-solvent hasto be miscible with water. An organic co-solvent and an organicnon-solvent are herein understood to be different organic solvents,which differ by the fact that the nylon salt is soluble in theco-solvent, or in the mixture of water and co-solvent, whereas the nylonsalt is not soluble in the non-solvent, or the mixture of thenon-solvent and water.

Isolation by crystallization is an inherently slow process due to slowcrystal growth rates, and is characterized by a very complex set-up ofunit operations. Furthermore, co-crystallization of mixed salts, i.e.salts prepared from a mixture of diamines and a mixture of dicarboxylicacids is a challenge in itself.

Precipitation with an organic non-solvent has the disadvantage thatrelative large amounts of the organic non-solvent have to be used andthat this non-solvent has to be recovered from water, resulting in largewaste streams and a complex process as well.

Precipitation induced by evaporation of water and any co-solvent presentcan result in formation of big lumps, which can lead to difficulties infull removal of the solvent at the end of the process, and to problemswith discharging the salt from the reactor, while big lumps needgrinding and classification for further use in further processes.

Spray drying involves atomization of a solution and drying of smalldroplets in a counter-current flow of heated inert gas. The processtypically results in a powder with relative small particles, and use ofinert gas is essential to avoid the explosion risk which such particlespose in air. For the inert gas generally nitrogen gas is used. Spraydrying is typically done in an apparatus comprising a very tall annularchannel, or drying tower, equipped with multiple high pressure nozzlesat the top of the channel, an inert gas stream, and a cyclone forseparation of the fines from the inert gas stream. In a spray dryingprocess, atomization of the solution into small droplets is critical,and also the droplet size needs careful control in order to obtain a drypowder at the bottom of the drying channel. For that purpose, nozzleswith small dimensions and solutions with low viscosity have to be used.The solutions must be clear and free of impurities. The solutions mustalso be relatively highly diluted to relatively low concentrations, wellbelow the saturation level of the salt, to prevent prematurecrystallization of the salt in the nozzles and to prevent blockage ofthe nozzles resulting from such premature crystallization. Due to therequirement of diluted solutions, large amounts of water need to beevaporated. This becomes even worse with nylon salts for semi-aromaticpolyamides, which generally have a much lower solubility than salts foraliphatic polyamides. Almost all of the energy input needed for theevaporation has to come from the heated inert gas. This requires largevolumes of heated gas to be circulated and heating with hot gas isinefficient and costly. Moreover, drying by heated inert gas requireslarge volume reactors and involves a complex process stream, since theinert gas needs filtering, heating, compressing and recycling,

Solution processes where nylon salts for semi-aromatic polyamides areprepared, comprising at least an aromatic diamine or an aromaticdicarboxylic acid, have the general disadvantage that the solubility ofthe salt is limited, and a large volume of water, or a mixture thereofwith another solvent is needed.

Nylon salts can also be prepared in solid state under dry conditions,i.e. without the use of a solvent or a melt, by dosing liquid diamine toa solid powder of dicarboxylic acid under conditions that thedicarboxylic acid and the salt resulting thereof always remain below themelting temperature of the dicarboxylic acid and of the salt. A problemis that such a process is difficult to be carried out at large scalewhile applying conditions that the reactor content retains in a powderystate.

Therefore, in view of the above, there is need for a process forpreparing a nylon salt that does not have the above disadvantages, or atleast so in less extent, and in particular for a process suitable forpreparing a semi-aromatic nylon salt in an effective and efficientmanner.

The aim of the present invention is to provide a process for preparing anylon salt in an effective and efficient manner.

This aim has been achieved with the process according to the invention,as mentioned in claim 1.

The process according to the invention is a flash process, wherein ahot, pressurized and concentrated aqueous mixture comprising a nylonsalt dissolved or dispersed in an aqueous medium, is flashed at elevatedtemperature, upon which water is evaporated and a granular material isobtained.

DETAILED DESCRIPTION

The effect of the process according to the invention, comprisingflashing of an aqueous salt mixture at elevated temperature, is that asolid salt in the form of a granular material is obtained in aneffective and efficient manner. With the process the granular nylon saltmaterial is obtained in a much faster and simpler manner thancrystallization or spray drying. Less water is needed, also because aslurry can be used, and no controlled droplet size atomization isrequired. The aqueous mixture can be flashed by a flash valve having arelatively large opening, without the need of spray nozzles with a smallopening, which significantly reduces the risk of blocking of the openingand also allows the use of slurries, which have a salt content wellabove the solubility level. Because of the high temperature and therelatively low water content, the water is evaporated adiabatically, orat least essentially so, without the need for applying a largecountercurrent of heated inert gas. This results in a fast and efficientdrying step and allows discharging of the aqueous mixture from apressure reactor in a relatively short time, much shorter compared tospray drying. No hot gas processing is needed, other than water vaporresulting from evaporation of the water from the flashed hot aqueousmixture, and no large volumes of inert gas have to be heated andprocessed. In order to reduce explosion risks, it is sufficient to fillthe flash chamber with inert gas. Any traces of oxygen still present,will be expelled with the water vapor. Because of the use of aconcentrated aqueous mixture, relatively small reactors can be used.Overall, reactor design and unit operations can be simplified and secondstream processing can be simplified to removal of hot steam andcondensation of water vapor. Surprisingly, even the lower solubility ofsemi-aromatic nylon salts is not a bottleneck to flash with sufficientlylow amount of water to achieve an essentially non-sticking powder. Alsosurprisingly a very good pelletizable product, with a relatively highbulk density, is obtained.

The process according to the invention comprises steps of:

-   -   a) providing in a pressure vessel a concentrated aqueous mixture        comprising at least 65 wt. % of salt components and at most 35        wt. % of the aqueous medium, the weight percentages (wt. %)        being relative to the total weight of the of the aqueous        mixture;    -   b) controlling the concentrated aqueous mixture in the pressure        vessel to have a temperature Tp of at least 150° C., thereby        providing a pressurized aqueous mixture;    -   c) controlling pre-condensation eventually occurring in the        concentrated aqueous mixture to remain at a level of at most 15        mole %;    -   d) discharging the pressure vessel by flashing the concentrated        aqueous mixture via a flash valve into a flash chamber, thereby        evaporating the aqueous medium in an amount sufficient to result        in a residual moisture content of at most 7.5 wt. %, and forming        a granular nylon salt material, wherein the wt. % is relative to        the weight of the granular nylon salt material.

The process according to the invention involves the use of an aqueousmixture, which can be either an aqueous solution or an aqueous slurry.The aqueous mixture is a concentrated aqueous mixture in that itcomprises at least 65 wt. % of salt precursors, in case of a solution,or salt components, in case of a slurry, and at most 35 wt. % of theaqueous medium. Herein the weight percentages (wt. %) are relative tothe total weight of the of the aqueous mixture.

The concentrated aqueous mixture used herein suitably is either:

-   -   i. an aqueous solution comprising at least 65 wt. % of salt        precursors and at most 35 wt. % of the aqueous medium;        -   or    -   ii. an aqueous slurry comprising a total amount at least 65 wt.        % of salt components and at most 35 wt. % of the aqueous medium.

The weight percentages (wt. %) being relative to the total weight of theof the aqueous mixture.

Salt precursors are herein understood to be the monomers from which thenylon salt is prepared, which monomers comprise a diamine, or a mixtureof at least two diamines, and a dicarboxylic acid, or a mixture of atleast dicarboxylic acids dissolved in the aqueous medium.

Salt components are herein understood to be the total of monomers fromwhich the nylon salt is prepared, and nylon salt resulting from themonomers. The slurry will comprise the nylon salt being dispersed in theaqueous medium slurry, and a remainder of monomers being dissolved inthe aqueous medium.

It is noted that the diamine used in the process for preparing the nylonsalt can be a single diamine, or a mixture of different diamines. Unlessspecifically noted otherwise, the single form ‘diamine’ has the meaningof including both a single diamine and a mixture of different diamines.Analogously, the dicarboxylic acid used in the process for preparing thenylon salt can be a single dicarboxylic acid, or a mixture of differentdicarboxylic acids. Unless specifically noted otherwise, the single form‘dicarboxylic acid’ has the meaning of including both a singledicarboxylic acid and a mixture of different dicarboxylic acids.

The expression “in the range of X-Y”, wherein X and Y represent lowerand upper limits, has the meaning that the range is from X up to andincluding Y. In other words, in said expression the values of bothlimits are included in the range.

With a granular material is herein understood a material consisting forat least 50 wt. % of particles with a particle size of at most 3 mm(millimeter). Such a material has a d50 of 3 mm or less. Herein theamount of particles with a particle size of at most 3 mm is determinedby sieving and weighing. The particle size for the d50 value,respectively the wt. % of particles with a particle size of at most 3 mmis determined by the method according to DIN 66-165 part 1 and 2.

The aqueous medium comprised by the slurry or solution used in theinventive process comprises at least water. It may suitably comprise aco-solvent, such as an organic solvent. Preferably the co-solvent is aliquid that is miscible with water. Suitably the liquid miscible withwater is an alcohol. Examples thereof include methanol and ethanol. Suchliquids miscible with water may be used for example to increase thesolubility of the salt in water. The liquid miscible with water may alsobe used otherwise, for example to partially precipitate the salt from anaqueous solution, thereby forming a slurry. Suitably, the co-solvent ispresent in an amount of 0-50 wt. %, and more particular 0-25 wt. %,relative to the total weight of water and co-solvent.

In a preferred embodiment, the aqueous medium consists of water. Thishas the advantage that no special precautions have to be taken forremoval, separation or recycling of any co-solvent. Furthermore, the useof water is generally sufficient since water appears to be a very goodsolvent also for semi-aromatic salts at elevated temperature, andbecause also a slurry can be used for the concentrated aqueous mixture,there is no need for complete dissolution at the high concentrationapplied.

By controlling the concentrated aqueous mixture in the pressure vesselto have a temperature Tp of at least 150° C., the concentrated aqueousmixture is also pressurized, thereby providing a pressurizedconcentrated aqueous mixture.

For preparing the pressurized concentrated solution different routes canbe applied. Two possible routes will be exemplified here.

Suitably, the aqueous solution is prepared by:

-   -   providing a slurry comprising salt components dispersed in an        aqueous medium in a pressure vessel; and    -   closing the pressure vessel and heating the slurry in the        pressure vessel to elevated temperature thereby increasing the        pressure in the pressure vessel, dissolving the salt components        and obtaining a clear solution.

The slurry may be provided in the pressure vessel, either by preparingthe slurry inside the pressure vessel, or by preparing the slurryoutside the pressure vessel and then charging the slurry into thepressure vessel. After a clear solution is obtained, if so needed orselected, the solution can be further concentrated to a higherconcentration, by evaporating a part of the aqueous medium, or heated tohigher temperature and brought to a higher pressure, or both. Hereinevaporating and heating might be done simultaneously or sequentially, inany combination of the two.

As an alternative, the aqueous solution may be prepared by

-   -   dissolving salt precursors in an aqueous medium, using an excess        of the aqueous medium, thereby obtaining a clear solution; and    -   evaporating the excess of the aqueous medium at elevated        temperature and elevated pressure, thereby obtaining a        pressurized concentrated solution.

Herein the aqueous medium in which the salt precursors are dissolved inthe first step, is used in an amount larger than needed for theconcentrated solution, thus comprising an excess, which is removed byevaporation in the second step.

Herein the elevated temperature is above 100° C., suitably between 100°C. and 170° C., and the elevated pressure is above atmospheric pressure,and will depend on the temperature and amount of aqueous medium presentat the different stages in this procedure. This procedure requires arelative large amount of the aqueous medium, more than in the procedureabove, but avoids difficulties in handling and heating a slurry anddetermining when the solution become completely clear.

The pressure vessel in which the concentrated aqueous solution isprepared might be the same as the pressure vessel from which thepressurized aqueous mixture is prepared, or it might be a different one,from which the concentrated aqueous solution is than charged into theother one.

The process for preparing the pressurized concentrated aqueous solutionis suitably carried out as follows:

-   -   mixing salt precursors with an aqueous medium in a weight ratio        salt precursors/aqueous medium of at least 10/90, preferably at        least 25/75, more preferably at least 50/50;    -   concentrating and evaporating to a weight ratio of at least        65/35 and at a temperature between 100° C. and 150° C., thereby        at least partially or fully dissolving the salt precursors in        the aqueous medium and forming a preliminary aqueous solution or        slurry having a temperature between 100° C. and 150° C.; and    -   heating the preliminary aqueous solution or slurry in a closed        vessel under pressure to a temperature Tp of at least 150° C.        and below 230° C., and sufficiently high to completely dissolve        the salt precursors in the aqueous medium, thereby resulting in        pressurized concentrated aqueous solution.

The slurry that can be used in the flash step in the salt preparationprocess according to the invention may be prepared by any suitableprocess wherein a concentrated aqueous slurry comprising nylon saltparticles dispersed in the aqueous medium is obtained. Preferably theslurry directly obtained from an aqueous solution comprising a diamineand a dicarboxylic acid dissolved in the aqueous medium. This can beaccomplished by the procedure, wherein the concentrated aqueous slurryis prepared by

-   -   i) preparing an aqueous solution by dissolving salt precursors        in an aqueous medium; and    -   ii) concentrating the aqueous solution by evaporating the        aqueous medium in an amount sufficient to result in partial        precipitation of the diammonium/dicarboxylate salt, thereby        obtaining the concentrated aqueous slurry comprising salt        particles dispersed in the aqueous medium.

For the preparation of the aqueous solution herein, suitably one of thetwo procedures described above is used. The advantage of preparing theslurry in this manner is that the amount of residual dicarboxylic acidin the granular salt material, if any, is low. With residualdicarboxylic acid is herein understood dicarboxylic acid that has notbeen neutralized by diamine in a salt formation reaction. This meansthat first the diamine and the dicarboxylic acid have to be completelydissolved in the aqueous medium, for example at elevated temperature,and then the slurry is formed, for example by partial evaporation of theaqueous solution and thereby concentrating the solution. Byconcentrating the solution, first the saturation level for the dissolvedsalt will be reached and upon further concentration going beyond thesaturation level, crystallization of the salt will occur, and formationof a slurry will occur.

The aqueous solution or aqueous slurry as used in the process accordingto the invention has a salt concentration of at least 65 wt. %. Theconcentration may well be as high as 95 wt. %, or even above, providedthat the aqueous mixture remains in a liquid state and can still beflashed. A low concentration has the advantage that it easier to obtaina clear solution. A high concentration has the advantage that a flashedproduct with a lower residual moisture content is obtained. Preferably,the salt concentration is in the range of 70-90 wt. %, still morepreferably 75-85 wt. %. Herein the salt concentration is calculated asthe weight percentage (wt. %) of total amount of diamine anddicarboxylic acid, relative to the total weight of the solution orslurry. For clarity: where in the process a mixture of diamines and/or amixture of dicarboxylic acid is used, for the calculation of the totalamount of diamine and dicarboxylic acid, all the diamines in the mixtureand all the dicarboxylic acids in the mixture have to be included.

The solution or slurry may comprise, next to the nylon salt,respectively the constituting diamine(s) and dicarboxylic acid(s), oneor more other components. In view of the high salt concentration, theamount of such other component or components shall be limited.Furthermore, such other component(s) should not inhibit the formation ofsolid salt particles in the flash process. Suitably, such othercomponents, if present at all, are selected from components that areuseful during the polymerization, or are useful in the polyamidecomposition to be prepared. Examples of such components are stabilizers,polymerization catalysts and inert additives, for example inorganicpigments. Suitably, the concentrated aqueous mixture comprises at most10 wt. %, more preferably at most 5 wt. %, of at least one othercomponent, wherein the weight percentage (wt. %) is relative to thetotal weight of the aqueous solution or slurry

During heating of the aqueous mixture to elevated temperature and duringthe step of controlling the aqueous mixture in the pressure vessel tohave a temperature above 150° C., thereby obtaining a pressurizedaqueous mixture, and optionally while maintaining the pressurizedaqueous mixture for a longer time at elevated temperature prior todischarging of the pressurized aqueous mixture from the pressure vesselby flashing into the flash chamber, eventually less or morepre-condensation may occur. Pre-condensation is herein understood to bethe formation of amide groups due to reaction of a part of the aminegroups and a part of the acid groups in the diamine and the dicarboxylicacid. The level of precondensation that has occurred, herein alsoreferred to as pre-condensation level, is herein calculated as the totalmolar amount of reacted amine groups and carboxylic acid groups relativeto the total molar amount of amine and carboxylic acid groups in thediamine and dicarboxylic acid monomers used in the preparation of thediammonium dicarboxylate salt.

The pre-condensation level is herein determined by ¹H-NMR measurementson the salt product; the relative amount X of reacted amine groups andcarboxylic acid groups is determined by integration of the NMR signalfor of α-(C)H hydrogen atoms in amide groups; the relative amount Y ofnon-reacted amine groups and carboxylic acid groups is determined byintegration of the signal α-(C)H atoms in the diamine and thedicarboxylic acid. The pre-condensation level is then calculated as thepercentage of X relative to (X+Y).

With an α-(C)H hydrogen atom is herein understood a hydrogen atom bondedto a carbon atom in α-position relative to respectively an amide group,an amine group or a carboxylic acid group. The α-(C)H atoms in amidegroups, i.e. the carbon bonded hydrogen atom in the group —C(H)—NH—CO—and the carbon bonded hydrogen atom in the group —NH—CO—C(H)—, have adifferent chemical shift in ¹H-NMR than carbon bonded hydrogen atom inthe group —C(H)—NH₂ and in the group —C(H)—COOH. In salts, based onmixtures of monomers comprising monomers not comprising an α-(C)H atom,the calculation is based on the ¹H-NMR signals from the monomerscomprising an α-(C)H atom. For example, in salts for semi-aromaticpolyamides based on aliphatic diamines and an aromatic dicarboxylicacid, the aromatic dicarboxylic acid does not comprise a α-(C)H atom.Herein the amount of non-reacted aliphatic diamine will berepresentative for the amount of non-reacted aromatic dicarboxylic acid,while the amount of reacted aliphatic diamine will be representative forthe amount of reacted aromatic dicarboxylic acid, and thus thedetermination and calculation as indicated above is directlyrepresentative for the level of pre-condensation occurred.

It is noted that a flash process, similar to which is used in thepresent invention for preparing a nylon salt, is known in the art forpreparing polyamide prepolymers. However, to obtain the prepolymers inthe form of a powder, and not as a sticky mass, the prepolymer must havea number average molecular weight of generally at least 1000, moreparticular at least 1500. This corresponds with a degree of condensationof at least 80%, more particular at least 90%. This is in particular thecase with semi-aromatic polyamides with mixed diamines and/or mixeddiacids. With aliphatic polyamides the prepolymers might have a somewhatlower number average weight e.g. at least 500, and a somewhat lowerdegree of condensation, e.g. 70%, while still obtaining a product inpowder form. Going further down in level of condensation, generally avery sticky or even a slimy or snotty product is obtained.

It has been observed in the present invention, that by keeping thepre-condensation level much lower, a salt in a granular form isobtained. The salt may contain some pre-condensed components, which areassumed to have the composition of a binary product comprising a diamineand a dicarboxylic acid each with one reacted group, and with oneremaining amine group and one remaining carboxylic acid group.

The process according to the invention requires controllingpre-condensation eventually occurring in the concentrated aqueousmixture to remain at a level of at most 15 mole %. This is in order toobtain the salt as a granular material.

Preferably, the pre-condensation level is at most 10%, more preferablyat most 7%. A lower pre-condensation level has the advantage that therisk of caking of the salt upon storage is lower.

The occurrence of pre-condensation can be observed in the flash processby monitoring the pressure in the pressure vessel. Due to thepre-condensation reaction, water is released. Due to the so-formedreaction water, which will add to the water already present in theaqueous mixture, the pressure will increase above the level which wouldbe normal for the amount of water already present in the aqueous mixturein combination with the temperature applied. The pressure mayfurthermore go up, as a result of a decrease in boiling point of theaqueous mixture due to the fact that the concentration of the salt ionsin the aqueous mixture is decreasing by reacting to pre-polymer. Inparticular when a highly concentrated aqueous mixture is used,comprising a relative large amount of salt components and a very lowamount of water the increase in pressure is visible already at a lowlevel of pre-condensation and can be significant at a high level ofpre-condensation.

With controlling the pre-condensation level is meant herein thatmeasures are taken by which the pre-condensation level stays below anindicated level. Controlling the pre-condensation to stay below a lowlevel is accomplished by applying a limited Tp and a limited residencetime t_(R) at elevated temperature. When the pre-condensation in theresulting salt appears to be too high, either Tp has to be lowered ort_(R) to be shortened, or both. Typically, either Tp is much lower thanthe temperature applied in the prepolymer-flash process mentioned aboveor t_(R) is much shorter than the residence time applied in the saidprepolymer flash process. The best results are obtained by applying botha lower Tp and a lower residence time at elevated temperature.

Preferably, the temperature Tp of the pressurized aqueous mixture in thereaction vessel, is kept below 230° C. A Tp in the range between 210° C.and 230° C. is suitably applied for salts based on aromatic dicarboxylicacids, while keeping the residence time rather short. Using such a hightemperature has the advantage that either a salt with a lower residualwater content is obtained, or that an aqueous mixture with a higheramount of the aqueous medium can be used.

For salts based on an aromatic dicarboxylic acid, using a temperatureprofile with a Tp of 230° C., the residence time is suitably kept atabout or better below 15 minutes, even better below 10 minutes. With aTp of 220° C., the residence time is suitably kept at about or betterbelow 30 minutes, even better below 20 minutes, while with a Tp of 210°C. the residence time is suitably kept at about or better below 90minutes, even better below 60 minutes.

During the process the temperature Tp is preferably kept in the range of150-210° C., more preferably in the range of 160-200° C. A lower maximumtemperature has the advantage that a longer residence time can beapplied, allowing longer time for preparing the heated and pressuredaqueous mixture in the pressure vessel, and/or for discharging thepressure vessel, and a higher temperature has the advantage that aproduct with a lower moisture content is obtained or a higher amount ofthe aqueous medium in the aqueous mixture can be used.

For salts based on monomers comprising aliphatic dicarboxylic acids,such a high Tp of above 200° C. may be applied but would require anextremely short residence time t_(R) to prevent pre-condensation tooccur in significant extent. For the preparation of aliphatic salts, orfor mixed salts comprising both aliphatic dicarboxylic acid and aromaticdicarboxylic acid, the temperature Tp is suitably even much lower andpreferably kept in the range of 150-180° C., or better 155-175° C. andfor pure aliphatic salts even more preferably in the range of 160-175°C.

The lower temperature as preferred for the aliphatic salts implies thatthe amount of aqueous medium in the aqueous mixture has to be furtherlimited to allow for obtaining a granular nylon salt material with asufficient low moisture content.

Preferably, the amount of the aqueous medium is at most 30 wt. %, morepreferably at most 25 wt. %. Correspondingly, the amount of saltcomponents is preferably at least 70 wt. %, more preferably at least 75wt. %.

For the aliphatic salts, the use of a Tp in the lower ranges incombination with a lower amount of aqueous medium still allows the useof an aqueous solution for the aqueous mixture, because of the highersolubility of aliphatic salts compared to their aromatic counterparts.Aliphatic salts, in particular those with shorter chain diamines, tendto crystallize in a crystal form including one or two molecules of waterper salt unit. Preparation of a slurry with a high salt content wouldcreate the risk of solidification of the concentrated mixture. This riskis reduced by using a higher amount of aqueous medium, and avoided byusing a solution. Preferably, for the aliphatic salts, the aqueousmixture is an aqueous solution, rather than an aqueous slurry. Also theamount of the aqueous medium is preferably at least 10 wt. %, morepreferably at least 15 wt. %. Correspondingly, the amount of saltcomponents is preferably at most 90 wt. %, more preferably at 85 wt. %.

An amount of aqueous medium of at most 30 wt. %, more particular at most25 wt. %, is also advantageously applied for the nylon salts based onaromatic dicarboxylic acids flashed at higher temperature, since thisresults in a granular nylon salt material having a lower residualmoisture content. Using a lower amount of aqueous medium may result inthe formation of a slurry due to the lower solubility of such salts, butthis is less critical. Also here the amount of the aqueous medium issuitably at least 10 wt. %, more particular at least 15 wt. %. This isadvantageous for obtaining a slurry with a lower viscosity.

The process according to the invention requires controlling theconcentrated aqueous mixture in the pressure vessel to have atemperature Tp of at least 150° C. By doing this in the pressure vesselthe aqueous mixture becomes pressurized. With controlling theconcentrated aqueous mixture to have said temperature Tp of at least150° C., is meant herein that measures are taken by which the aqueousmixture retains such a temperature of at least 150° C., or by which theaqueous mixture attains such a temperature. For example, in case theconcentrated aqueous mixture is prepared outside the pressure vessel andcharged under pressure into the pressure vessel while already having atemperature of at least 150° C., the measures may exist in keeping thetemperature Tp of at least 150° C. Also when the aqueous mixture isprepared inside the pressure vessel at a temperature of at least 150° C.under pressure, the measures may exist in keeping the temperature Tp atat least 150° C. The measures may also exist in first heating theaqueous mixture in the pressure vessel to a temperature of at least 150°C., and then retaining the temperature at least at 150° C. while keepingthe pressure vessel closed. These measures may suitably be applied whenthe concentrated aqueous mixture is first prepared outside the pressurevessel and charged to the pressure vessel at a temperature below 150°C., or when the aqueous mixture is prepared inside the pressure, andhaving a temperature below 150° C., before being heated to at least 150°C.

In the process according to the invention a pressurized and concentratedaqueous mixture having a temperature of at least 150° C. is flashed froma pressure vessel into a flash chamber thereby providing a granularnylon salt material. The process has to be carried out such that theaqueous medium is evaporated in the flash step (step d) in an amountsufficient to result in a residual moisture content of at most 7.5 wt. %in the granular nylon salt material, wherein the wt. % of residualmoisture content is relative to the weight of the granular nylon saltmaterial. This requirement can be accomplished by selecting theappropriate amount of aqueous medium in combination with the temperatureTp applied for pressurized aqueous solution. As an indication, theaqueous medium is suitably present in an amount, depending as follows onTp:

-   -   of at most 20 wt. %, while Tp is in the range from 150° C. to        170° C.; or    -   of at most 25 wt. %, while Tp is in the range from 170° C. to        190° C.; or    -   of at most 30 wt. %, while Tp is in the range from 190° C. to        210° C.; or    -   of at most 35 wt. %, while Tp is in the range from 210° C. to        230° C.

In a preferred embodiment of the process according to the invention,wherein the nylon salt is an aliphatic salt, i.e. the salt componentstherein being all aliphatic components, comprising an aliphatic diamine,or a mixture of at least two aliphatic diamines, and an aliphaticdicarboxylic acid, or a mixture of at least two aliphatic dicarboxylicacids, the aqueous medium is present in an amount of at most 20 wt. %,relative to the total weight of the aqueous mixture, and the temperatureTp is in the range from 150° C. to 170° C.

To be able to flash the aqueous mixture from the pressure vessel intothe flash chamber in a sufficiently short time, it is advantageous tohave a minimum pressure difference between the pressure vessel and theflash chamber. Preferably, the pressure difference ΔP between thepressure vessel and the flash chamber is at least 2 bar, more preferablyat least 5 bar. Using a higher water content, for example 15 wt. % ormore, in combination with at higher temperature, such as 170° C. ormore, the pressure in the pressure vessel will be at least 5 barg, or 6bara. With an atmospheric pressure (1 bara) in the flash chamber, thedifference ΔP will then also be at least 5 bar. To attain a differenceΔP at a temperature below 160° C., the pressure in the pressure vesselmay be raised by applying pressure with nitrogen gas or overheatedsteam.

The process according to the invention is suitably carried out in anapparatus comprising a pressure vessel equipped with a flash valve, anda flash chamber. The flash valve is herein positioned between thepressure vessel and the flash chamber, thus allowing by opening of theflash valve for discharging of the aqueous mixture from the pressurevessel into the flash chamber. The forming of granular salt material bythe flash process according to the invention is supposed to proceed asfollows: the salt solution or salt slurry is flashed into a flashchamber. Upon flashing, the pressure is released, droplets are formedand the aqueous medium evaporates adiabatically or essentially so, whichoccurs very fast, upon which the salt solidifies and the nylon saltparticles are formed. The evaporated water is removed as steam, whilethe nylon salt particles settle down, thereby obtaining the granularsalt material.

The process according to the invention may be carried out, for example,batch wise or in a continuous manner. On small scale the process isconveniently carried out in a batch autoclave, whereas on industrialscale a continuous evaporator, discharging to a continuously operatedflash nozzle and flash chamber, may be more convenient. Continuousoperation has the additional advantages of allowing shorter residencetimes of the salt solution at elevated temperatures and reduced size ofequipment for a given production capacity. Design guides for continuousevaporators, such as thin film evaporators can readily be found instandard text books, such as Perry's Chemical Engineers' Handbook 7thedition section 11-13.

In the flash step, the aqueous medium is suitably evaporated in anamount, sufficient to result therein that the granular salt material hasa residual moisture content of at most 7.5 wt. %. Preferably, theresidual moisture content is at most 5.0 wt. %, more preferably at most4.0 wt. %, still more preferably at most 2.0 wt. %. Herein the weightpercentage (wt. %) is relative to the total weight of the granular saltmaterial including the residual moisture. The advantage of a lower watercontent is reduced risk of caking upon storage. With moisture is hereinunderstood water, and where applicable any co-solvent, retained in thegranular salt material obtained after the flash process. The residualmoisture content is measured by determining the weight loss of thegranular salt material upon drying of the salt during 24 hours at 105°C. and under a vacuum of 100 mmHg. The residual moisture content iscalculated from the weight loss expressed as weight percentage relativeto the weight of the granular salt material before drying.

The flash chamber comprised by the apparatus in which the flash processis carried out, is suitably operated at a pressure, which allows theaqueous medium to be evaporated adiabatically, or essentially so, uponflashing form the pressure vessel. The pressure in the flash chambercan, most practically, be chosen to be equal to atmospheric pressure.The pressure in the flash chamber may also be above, or belowatmospheric pressure, as long as the pressure difference with thepressure in the pressure vessel is sufficient to allow for adiabaticevaporation, or essentially so, of the aqueous medium from the flashedaqueous mixture. Advantageously, the pressure in the flash chamber isbelow atmospheric pressure. This will enhance the evaporation and resultin a drier salt product.

The flash chamber, or at least the side wall thereof, may be heated toavoid condensation of vaporized water into liquid water flowing downfrom the wall, and thereby to avoid condensed water to get into contactwith the granular salt product.

In the process according to the invention, in principle any nylon saltcan be prepared provided that the salt can be dissolved in an aqueousmedium and an aqueous solution of the constituting diamine(s) and thedicarboxylic acid(s) can be prepared. Forming of a granular saltmaterial is generally no issue since dry or substantially dry nylonsalts are typically solid materials at room temperature.

The salt suitably is a salt of a single diamine and a singledicarboxylic acid. The salt may also be a mixed salt, made from multiplecomponents. The mixed salt is suitably made of components comprising atleast two diamines and one dicarboxylic acid, or one diamine and atleast two dicarboxylic acids, or even at least two diamines and at leasttwo dicarboxylic acids. The mixed salt may eventually comprise three ormore diamines and or three or more dicarboxylic acids.

Suitably, the diamine used for the salt in the process according to theinvention is selected from aliphatic diamines, or aromatic diamines, orany combination thereof. Examples of suitable aliphatic diamines areethylene diamine, 1,3-propane-diamine, 1,4-diaminobutane,1,5-pentanediamine, 2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine, isophoronediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine,1,11-undecanediamine, 1,12-dodecanediamine, 1,13-tridecanediamine,1,14-tetradecanediamine, 1,15-pentadecanediamine,1,16-hexadecanediamine, 1,17-heptadecanediamine, 1,18-octadecanediamine,1,19-nonadecanediamine, and 1,20-eicosanediamine. Examples of aromaticdiamines are 1,4-diaminobenzene, 1,3-diaminobenzene,1,4-naphthalenediamine, 1,5-naphthalenediamine 2,6-naphthalenediamine,meta-xylenediamine and para-xylenediamine.

Preferably, the diamine comprises at least an aliphatic C2-C10 diamine,i.e. an aliphatic diamine with from to 2 up to and including 10 carbonatoms. The advantage thereof is that the salt resulting from the diaminehas a higher solubility in water. More preferably, the diamine comprisesone or more than one aliphatic C2-C10 diamine in an amount of at least50 mole %, relative to the total molar amount of diamine. Still morepreferably, the amount of aliphatic C2-C10 diamine is at least 75 mole%, relative to the total molar amount of diamine.

Even more preferably, the diamine comprises one or more than onealiphatic C4-C6 diamine in an amount of at least 50 mole %, relative tothe total molar amount of diamine. Still more preferably, the amount ofaliphatic C4-C6 diamine is at least 75 mole %, relative to the totalmolar amount of diamine. The advantage thereof is that lowertemperatures can be used to flash to a dry salt, and polymerizationbefore flashing is suppressed.

The aliphatic C4-C6 diamine suitably consists of a diamine selected from1,4-diaminobutane, 1,5-pentanediamine, 2-methyl-1,5-pentanediamine and1,6-hexanediamine, or any mixture thereof; preferably selected from1,4-diaminobutane, 1,5-pentanediamine and 1,6-hexanediamine, or anymixture thereof.

Suitably, the dicarboxylic acid is selected from aromatic dicarboxylicacid, or aliphatic dicarboxylic acid, or any combination thereof.Suitable examples of aromatic dicarboxylic acids are terephthalic acid,isophthalic acid, 2,6-naphthalene dicarboxylic acid, 4,4′-diphenyldicarboxylic acid. For the aliphatic dicarboxylic acids suitableexamples are oxalic acid, succinic acid, 1,5-pentanedicarboxylic acid,adipic acid, 1,7-heptanedioic acid, 1,8-octanedioic acid, azaleic acid(1,9-nonanedioc acid), sebacic acid (1,10-decanoic acid),1,11-undecanoicacid, 1,12-dodecanoic acid, 1,13-tridecanoic acid, 1,14-tetradecanoicacid, 1,15-pentdecanoic acid, 1,16-hexadecanoic acid, 1,17-heptadecanoicacid, 1,18-octadecanoic acid, 1,19-nonadecanoic acid, and1,20-eicosanedioic acid.

Preferably, the dicarboxylic acid comprises a C4-C10 aliphaticdicarboxylic acid, i.e. an aliphatic dicarboxylic acid with from to 4 upto and including 10 carbon atoms. The advantage thereof is that the saltresulting from the dicarboxylic acid has a higher solubility in water,allowing the use of a higher salt concentration and less water to beevaporated.

Also preferably, the dicarboxylic acid comprises an aromaticdicarboxylic acid. The advantage of this selection is that thetemperature conditions applied in the flash drying can be varied over awider range.

Examples of salts that can be prepared with the process according to theinvention include the following semi-aromatic salts: XT, XI, XT/XI,XT/X6, XT/Y6 and XT/YT, and any combinations thereof, wherein Trepresents terephthalic acid, I represents isophthalic acid, 6represents adipic acid, and X and Y represent diamines. In a preferredembodiment X and Y are different diamines, independently from each otherchosen from aliphatic diamines from the group consisting of ethylenediamine, 1,4-diaminobutane, 1,5-pentanediamine,2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine,1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine.

Other examples of salts that can be prepared with the process accordingto the invention include the following aliphatic salts: 46, 66, 46/66,66/68, and 410, and any combinations thereof.

The salts may also be mixed salts, for example comprising a mixture ofcomponents from any of the mentioned semi-aromatic salts XT, XI, XT/XI,XT/X6, XT/YT and XT/YT and any of the aliphatic salts 46, 66, 46/66,66/68, and 410.

In a preferred embodiment of the invention, the dicarboxylic acid in thesalt comprises terephthalic acid, for example in an amount of at least50 mole %, more particular at least 75 mole %, or even consists ofterephthalic acid.

In a more preferred embodiment, the dicarboxylic acid consists for atleast 75 mole % of terephthalic acid, and the diamine consists for atleast 75 mole % of 1,6-hexamethylediamine (HMDA) and/or1,4-diaminobutane (DAB) and/or 1,5-pentanediamine.

Preferably the diamine comprises 1,6-hexamethylediamine (HMDA) and1,4-diaminobutane (DAB) in a molar ratio DAB/HMDA in the range of25:75-45:55, or even better 30:70-40:60, as this gives rise to apolyamide with eutectic melting point.

Also preferable, the diamine comprises 1,6-hexamethylediamine (HMDA) and1,5-pentanediamine (PD) in a molar ratio PD/HMDA in the range of30:70-70:30, or even better 40:60-60:40.

Even more preferred the dicarboxylic acid consists for 95-100 mole % ofterephthalic acid, and the diamine consists for at least 95-100 mole %of 1,6-hexamethylediamine (HMDA), 1,4-diaminobutane (DAB) or1,5-pentanediamine. Suitably, the corresponding polyamides are eitherhomopolymers PA-4T, PA-5T or PA-6T, or copolymers thereof such asPA4T/5T, PA-4T/6T and PA-5T/6T, or PA-41/5T/61.

In the process according to the invention, the dicarboxylic acid and thediamine do not need to be present in exact equimolar amounts. Suitably,the dicarboxylic acid and the diamine are present in a molar ratio inthe range of 0.95:1-1.05:1, preferably 0.98:1-1.02:1, more preferably0.99:1-1.01:1.

The invention also relates to a nylon salt. The nylon salt according tothe invention is a granular salt material comprising a diammoniumdicarboxylate salt obtainable by flash drying.

Suitably, the granular salt material according to the invention has aparticle size distribution with a median particle size (d50) in therange of 50-3000 μm, in the range of 200-1000 μm, more preferably400-750 μm. Herein the particle size distribution is measured with lasergranulometry by the method according to ISO 13320-1 at 20° C.

The granular salt material according to the invention has furtherinteresting properties such as high density and low compressibility. Thecompressibility is determined by comparing the aerated bulk density(ABD) and the tapped bulk density (TBD). Each of the aerated bulkdensity (ABD) and the tapped bulk density (TPB) are measured by themethod according to ASTM D6393. Suitably, the compressibility, expressedby the ratio of (TBD-ABD)/TBD*100%, is at most 25%.

Suitably, the granular salt material has a tapped bulk density in therange of 350-750 kg/m³; preferably the tapped bulk density in the rangeof 450-700 kg/m³.

The granular salt material obtained by the flash process can be easilypelletized, optionally using a few percent of additional water asbinder. The granular salt material, as well as the pelletized productmade thereof can be polymerized into a polyamide polymer using anysuitable polymerization process. In particular, the salt can polymerizedby direct solid state polymerization, by applying a suitable heatingprofile, in which the temperature remains below the melting temperatureof the salt. Preferably, during the direct solid state polymerization,the granular salt material is heated to elevated temperature, andwherein said temperature is kept below the melting temperature of thesalt minus 10° C., preferably below the melting temperature of the saltminus 15° C.

With the term melting temperature of the salt is herein understood thetemperature, measured by the DSC method according to ISO-11357-1/3,2011, on pre-dried samples in an N2 atmosphere with heating and coolingrate of 10° C./min. Herein Tm has been calculated from the peak value ofthe highest melting peak in the first heating cycle. Herein thegranulate salt material is dried during 24 hours at 105° C. and under avacuum of 100 mmHg.

The invention also relates to a process for preparing a nylon polymer.The process comprises direct solid state polymerization (DSSP) of adiammonium dicarboxylate salt (i.e. a nylon salt), wherein the salt is agranular salt material prepared by flash granulation, or a pelletizedproduct made thereof.

The granulate salt material according to the invention may also be usedin a process wherein a diammonium dicarboxylate salt is polymerized in aliquid state. Herein the diammonium dicarboxylate salt suitablycomprises a granulate salt material prepared by flash granulation andthe granulate salt material is either molten or dissolved. For example,a 66 salt prepared by flash granulation can be advantageously used in aconventional melt polymerization process, to produce polyamide-66 or acopolymer thereof. Another example is the polymerization of such a saltin a solution process in an aqueous solution to prepare a prepolymer,followed by isolation of the prepolymer and subsequent solid state postcondensation to convert the prepolymer in a polyamide polymer.

The invention is further illustrated with the following examples andcomparative experiments.

Methods

Aerated Bulk Density (ABD) and Tapped Bulk Density (TBD)

The ABD and TBD were measured by the method according to ASTM D6393-08(“Standard Test Method for Bulk Solids Characterization by CarrIndices”, ASTM International, West Conshocken, Pa., DOI:10.1520/D6393-08) with a Hosokawa Powder Tester at 20° C.

Residual Water Content

The residual water content of a salt was determined by drying of thesalt during 24 hours at 105° C. and under a vacuum of 100 mmHg, i.e.equal to an absolute pressure of 0.01 bar. The weight of the salt wasmeasured before and after drying. The residual water content wascalculated from the weight loss expressed as weight percentage relativeto the weight of the granular salt material before drying.

Compositions of Salts and Polymers

The composition of the salt with regard to ratio of diamines wasdetermined by ¹H-NMR. The ratio of dicarboxylic acid/diamine wascalculated form the excess of diamine determined by titration of a saltsolution in water. The number of end groups, i.e. residual aminefunctional groups and residual carboxylic acid functional groups wasdetermined by ¹H-NMR

Preparation of nylon salt by flashing: general procedure.

The preparation process was carried out using a 2.5 liter autoclaveequipped with a heating mantle, a lid, a stirrer, a nitrogen gas inlet,an inspection glass, a distillation valve and a pressure meter at thetop, and a flash valve at the bottom. Below the autoclave, a 10 literflash chamber was positioned, the flash chamber comprising an openingpositioned closely to the flash valve and another opening for gasinlet/outlet. The autoclave was filled with diamine, dicarboxylic acidand water, mixed and inertisized three times with nitrogen of 10 barg.Then the mixture was heated up, with a heating ramp of 2° C./min, uponwhich the salt components dissolved in the water, resulting in a clearsalt solution. During further heating up, the water was distilled at apreset pressure and temperature until a solution or slurry with acertain concentration was reached. Then the reactor was closed byclosing the distillation valve and further heated up to a highertemperature. The pressure at that temperature was monitored. The natureof the aqueous mixture was visually inspected, in order to determinewhether it was a solution or a slurry. In most of the examples, themixture inside the reactor showed to be a suspension of fine saltcrystals. Prior to flashing of the slurry, the flash chamber wasinertisized by leading nitrogen gas through the flash chamber. Whereapplicable the flash chamber was preheated in an oven set at apredetermined temperature. Then, the total amount of the aqueous mixturewas discharged from the autoclave by opening the flash valve and flashedinto the inertisized flash chamber. Upon flashing, the water present inthe aqueous mixture, or the larger part of it, evaporated immediatelyand escaped via the gas inlet/outlet. The salt stayed behind in theflash chamber.

Salt Compositions

For the diamine in Comparative Experiments A-C and Examples I-VI, amixture of 1,4-butanediamine and hexamethylenediamine in a mole ratio ofabout 40/60 was used. For examples VII and VIII a mixture of1,5-pentanediamine and hexamethylenediamine in a mole ration of about40/60 was used, while in example IX pure 1,5-pentanediamine was used.For the dicarboxylic acid, terephthalic acid was used in all cases. Forthese raw materials, industrial grades were used.

Results

Different experiments were carried out using different saltconcentrations and/or different temperature profiles. The observationsand results for the different experiments are shown in Tables 1A and 1Bbelow.

Pelletization

For the preparation of pellets, two metal molds were used, each with acylindrical hole, one with a diameter of 5 mm and a height of 25 mm, theother one with a diameter of 13 mm and a height of 32 mm. The mold wasfilled with granular salt material. Then the mold was placed in a pressequipped with a metal stamp of appropriate diameter. Then the granularsalt material was compressed by first lowering the stamp manually andthen pressing the stamp with a pressure of 60 kN/cm2.

TABLE 1A Observations and results for the Comparative Experiments CE/ACand Examples EX-II, V and VI CE-A CE-B CE-C EX-II EX-V EX-VI Initialconcentration 50 65 65 70 70 70 (wt. %) Initial temperature 50 50 50 5050 50 (° C.) Visual appearance of clear slurry slurry slurry slurryslurry mixture Temperature of heated 170 200 220 180 190 200 solution (°C.) Visual appearance clear clear clear clear clear clear Concentrationafter 60 69 70 82 82 82 evaporation (wt. %) Visual appearance of ClearClear Clear Slurry Slurry Slurry mixture Further heating step No No NoYes No No Temperature at start of 170 200 220 190 190 200 flash step (°C.) Waiting time at flash 0 0 15 0 15 15 temperature (min) Pressure(barg) 5 5 >25 7 7 11.5 Total time Q (minutes) 60 60 85 60 75 80 (note(a)) Visual appearance of clear clear clear slurry slurry slurry mixtureTemperature of wall Rt 150 150 150 150 150 flash chamber (° C.) Durationof flash step 10 10 — 10 10 1 (seconds) Appearance product NG NG n.a. OKOK OK (note (b)) (note (d) Pre-condensation n.a. 1.6 2.0 2.9 level (%)(note (d) Aerated bulk n.a. 490 490 density (g/l) (note (d) Tapped bulkn.a. 590 590 density (g/l) (note (d) Residual water 22 11 n.a. 4.5content (wt. %) (note (d) Cleanliness reactor 4 3 n.a. 5 4 3 (note (c))(note (d) Observations and results for the Comparative ExperimentsCE-A/C and Examples EX-II, I, III-IV and VII-IX EX-VII EX-VIII EX-IXNote Note Note EX-III EX-IV EX-I (e) (e) (f) Initial concentration 70 7065 65 65 65 (wt. %) Initial temperature 50 50 50 50 50 50 (° C.) Visualappearance slurry slurry slurry slurry Slurry Slurry of mixtureTemperature of 180 180 215 180 180 180 heated solution (° C.) Visualappearance clear clear clear Clear Clear Clear Concentration after 82 8270 71 83 70 evaporation (wt. %) Visual appearance Slurry Slurry ClearSlurry Slurry Clear of mixture Further heating step Yes Yes No Yes YesYes Temperature at start 200 210 215 215 200 215 of flash step (° C.)Waiting time at flash 0 0 0 0 0 0 temperature (min) Pressure (barg) 1011 15 15 10 15 Total time Q 60 65 70 70 60 70 (minutes) (note (a))Visual appearance slurry slurry clear clear slurry clear of mixtureTemperature of wall 150 150 150 150 150 150 flash chamber (° C.)Duration of flash 10 1 10 10 10 10 step (seconds) Appearance product OKOK OK OK OK OK (note (b)) Pre-condensation 2 4 3.5 2 3.6 level (%)Aerated bulk 480 density (g/l) Tapped bulk 590 density (g/l) Residualwater 3 1 <1 <1 2.5 <1 content (wt. %) Cleanliness 3 3 3 3 3 3 reactor(note (c)) Notes for Tables 1A and 1B: (a) Total time from first heatedtemperature, before, during, after evaporation, plus further heating,until start of flashing (b) Appearance product: NG = not good, Stickysalt lumps, big crystals; MO: moderate; OK = good: surface dry,non-sticky powder; other? (c) Cleanliness reactor: 1 = very bad, veryhard to clean; 2 = not good, residues of prepolymer, difficult to cleanwith water; 3 = acceptable, some prepolymer left in the reactor, buteasy to clean with water; 4 = good: residues of salt, but easy to cleanwith water; 5 = excellent: hardly any visible residues or even none; (d)Experiment was interrupted because of observed pressure increase,significant above the level of 16-17 bars, estimated for the appliedtemperature and water content (e) Experiment with a mixture of1,5-pentanediamine, hexamethylenediamine (40/60 molar ratio) andterephthalic acid (f) Experiment with 1,5-pentanediamine andterephthalic acid

The results for Examples II-IV show that using a higher temperature forthe flash step, a product with a lower water content is obtained. Theresults also show that using a higher temperature for the flash step, aproduct with a higher degree of pre-condensation is obtained.Comparative experiments CE-A and CE-B show that if the flash step iscarried out at a relatively low temperature with a relatively high watercontent, a non-granular product with a too high residual water contentis obtained. Comparative experiments CE-C shows that if the flash stepis carried out at a relatively high temperature with a relatively longresidence time, a significant pressure increase is observed, which isconsidered to be indicative for extensive pre-condensation to haveoccurred. Examples VII and VIII show that very similar results areobtained using the 1,5-pentanediamine, hexamethylenediamine (about 40/60molar ratio) with terephthalic acid mixtures, while example IX showsthat pure 1,5-pentanediamine terephthalate can also be used in thisprocess. The pellets from the Examples according to the invention soproduced showed a good integrity and a high strength.

The invention claimed is:
 1. A process for preparing a granular nylonsalt material comprising steps of: (a) providing in a pressure vessel aconcentrated aqueous mixture comprising at least 65 wt. % of saltcomponents and at most 35 wt. % of an aqueous medium, the weightpercentages (wt. %) being relative to the total weight of the aqueousmixture; (b) controlling temperature of the concentrated aqueous mixturein the pressure vessel so that the concentrated aqueous mixture has atemperature Tp of 150° C. to 230° C.; (c) controlling pre-condensationeventually occurring in the concentrated aqueous mixture so thatpre-condensation therein remains at a level of at most 15 mole %; and(d) discharging the concentrated aqueous mixture from the pressurevessel by flashing the concentrated aqueous mixture at the temperatureTp of 150° C. to 230° C. via a flash valve into a flash chamber toevaporate the aqueous medium in an amount sufficient to result in aresidual moisture content of at most 7.5 wt. % and thereby directly forma granular nylon salt material, wherein the wt. % of the residualmoisture content is relative to the weight of the granular nylon saltmaterial.
 2. The process according to claim 1, wherein the concentratedaqueous mixture is: (i) an aqueous solution comprising a diamine, or amixture of at least two diamines, and a dicarboxylic acid, or a mixtureof at least two dicarboxylic acids, all together referred to as saltprecursors, dissolved in the aqueous medium; or (ii) an aqueous slurrycomprising a diammonium dicarboxylate salt dispersed in the aqueousmedium.
 3. The process according to claim 2, wherein the concentratedaqueous mixture is an aqueous solution prepared by: dissolving saltprecursors in an aqueous medium, using an excess of the aqueous medium,thereby obtaining a diluted solution; and evaporating the excess of theaqueous medium at elevated temperature and elevated pressure, therebyobtaining a pressurized concentrated solution.
 4. The process accordingto claim 2, wherein the concentrated aqueous mixture is an aqueoussolution prepared by: providing a slurry comprising salt componentsdispersed in an aqueous medium in a pressure vessel; and closing thepressure vessel and heating the slurry in the pressure vessel toelevated temperature thereby increasing the pressure in the pressurevessel, dissolving the salt components and obtaining a pressurizedconcentrated solution.
 5. The process according to claim 2, wherein theconcentrated aqueous mixture is a concentrated aqueous slurry directlyobtained from an aqueous solution comprising salt precursors dissolvedin the aqueous medium.
 6. The process according to claim 5, wherein theconcentrated aqueous slurry is prepared by: preparing an aqueoussolution by dissolving salt precursors in an aqueous medium; andconcentrating the aqueous solution by evaporating the aqueous medium inan amount sufficient to result in partial precipitation ofdiammonium/dicarboxylate salt, thereby obtaining the concentratedaqueous slurry comprising salt particles dispersed in the aqueousmedium.
 7. The process according to claim 1, wherein step (d) ispracticed by flashing the concentrated aqueous mixture such that theaqueous medium is evaporated in an amount sufficient to result in aresidual moisture content of at most 5.0 wt. %, relative to the weightof the granular nylon salt material.
 8. The process according to claim1, wherein the aqueous medium consists of water.
 9. The processaccording to claim 1, wherein the aqueous medium is present in an amountof at most 20 wt. %, relative to the total weight of the aqueousmixture, and wherein the temperature Tp is in a range from 150° C. to170° C.
 10. The process according to claim 1, wherein the aqueous mediumis present in an amount of at most 25 wt. %, relative to the totalweight of the aqueous mixture, and wherein the temperature while Tp isin a range from 170° C. to 190° C.
 11. The process according to claim 1,wherein the aqueous medium is present in an amount of at most 30 wt. %,relative to the total weight of the aqueous mixture, and wherein thetemperature while Tp is in a range from 190° C. to 210° C.
 12. Theprocess according to claim 1, wherein the aqueous medium is present inan amount of at most 35 wt. %, relative to the total weight of theaqueous mixture, and wherein the temperature while Tp is in a range from210° C. to 230° C.
 13. The process according to claim 1, wherein thenylon salt is an aliphatic salt, the salt components therein being allaliphatic components, comprising an aliphatic diamine, or a mixture ofat least two aliphatic diamines, and an aliphatic dicarboxylic acid, ora mixture of at least two aliphatic dicarboxylic acids, and wherein theaqueous medium is present in an amount of at most 20 wt. %, relative tothe total weight of the aqueous mixture, and wherein the temperature Tpis in a the range from 150° C. to 170° C.
 14. The process according toclaim 1, wherein the nylon salt is a semi-aromatic salt.
 15. The processaccording to claim 14, wherein the semi-aromatic salt comprisesterephthalic acid.
 16. A nylon salt which is a granular salt materialprepared by flash granulation by the process according to claim
 1. 17.Compressed pellets comprised of the nylon salt according to claim 16.18. A granular salt material which is prepared by the process accordingto claim 1.